Added a dark/light theme toggle button to the UI, allowing users to switch between the existing dark theme and a new light theme.

1. Light theme CSS variables — Added a [data-theme="light"] block after :root with a full set of overrides:

   • Background: #f8fafc (near-white)
   • Surface: #ffffff
   • Surface hover: #f1f5f9
   • Text primary: #0f172a (near-black for high contrast)
   • Text secondary: #64748b
   • Border color: #e2e8f0
   • Assistant message bg: #f1f5f9
   • Shadow: lighter rgba(0,0,0,0.1) drop shadow
   • Welcome background: #dbeafe
   • Primary/accent colors unchanged (#2563eb)

2. Body transition — Added transition: background-color 0.3s ease, color 0.3s ease to body so theme switches animate smoothly.

3. .theme-toggle button styles — New styles for a fixed, circular button positioned at top: 1rem; right: 1rem:

   • 40x40px circle with border and surface background
   • Hover: scales up 10%, highlights with primary color border
   • Focus ring using --focus-ring variable for accessibility
   • Active state: slight scale-down press effect
   • Sun/moon SVG icons absolutely positioned within the button, animated with opacity and rotation transitions
   • In dark mode (default): sun icon visible, moon icon hidden
   • In light mode ([data-theme="light"]): moon icon visible, sun icon hidden

• Added a <button class="theme-toggle" id="themeToggle"> element just before the closing </body> tag
• Contains two inline SVGs:
  • .icon-sun — rays + circle sun, shown in dark mode (click to switch to light)
  • .icon-moon — crescent moon path, shown in light mode (click to switch to dark)
• aria-label="Toggle theme" and title attribute for accessibility and tooltip

1. initTheme() — Reads localStorage.getItem('theme'); if 'light', sets data-theme="light" on <html> so the chosen theme persists across page reloads.

2. toggleTheme() — Checks current data-theme on <html>:

   • If light: removes attribute (reverts to default dark), saves 'dark' to localStorage
   • If dark: sets data-theme="light", saves 'light' to localStorage

3. setupEventListeners() — Wires themeToggle.addEventListener('click', toggleTheme).

4. DOMContentLoaded handler — Grabs themeToggle element and calls initTheme() before session setup.

No frontend changes were made for the testing feature. This enhancement targeted the backend testing infrastructure only.

• Added shared test data constants: SAMPLE_ANSWER, SAMPLE_SOURCES, SAMPLE_SESSION_ID, SAMPLE_COURSES
• Added mock_rag fixture — a fully mocked RAGSystem with pre-configured return values
• Added make_test_app(rag_system) helper — builds a minimal FastAPI app with the same /api/query, /api/courses, and /api/session/{id} endpoints but without the static file mount, avoiding file-system dependency on ../frontend/
• Added test_app and client fixtures that compose make_test_app + starlette.testclient.TestClient

16 tests across three classes:

• TestQueryEndpoint — POST /api/query: status codes, response shape, session creation vs reuse, RAG delegation, 500 on error, 422 on missing field
• TestCoursesEndpoint — GET /api/courses: status code, response shape, mocked data, 500 on error
• TestSessionEndpoint — DELETE /api/session/{id}: status code, response body, correct ID forwarded to session_manager.clear_session

• Added httpx>=0.27.0 to [dependency-groups] dev (required by starlette.testclient.TestClient)
• Added [tool.pytest.ini_options] section:
  • testpaths = ["backend/tests"] — pytest discovers tests without specifying the path manually
  • pythonpath = ["backend"] — backend modules importable without sys.path hacks in each test file
  • addopts = "-v" — verbose output by default